Wave Equation in One Dimension: Vibrating Strings and Pressure Waves

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1 BENG 1: Mathematical Methods in Bioengineering Lecture 19 Wave Equation in One Dimension: Vibrating Strings and Pressure Waves References Haberman APDE, Ch. 4 and Ch

14 BENG 1 Lecture 17 M. Intaglietta The one dimensional wave equation. The vibrating string as a boundary value problem Given a string stretched along the x axis, the vibrating string is a problem where forces are exerted in the x and y directions, resulting in motion in the x-y plane, when the string is displaced from its equilibrium position within the x-y plane, and then released. The free body diagram of an element of string of length s subjected to a tension T is shown. The string material has density ρ. The equation of motion is obtained by applying Newton s second law of motion to the element of length s in both directions. For the x direction (and ignoring the torsional effects due to the applied torque): x ( T + T)cos( θ + θ) Tcosθ = ρa s t and in the y direction: y ( T + T)sin( θ + θ) Tsinθ = ρa s t Where the bar over the partial derivative signifies the average acceleration over the element s. A is the cross section of the string, assumed constant and equal to 1. Dividing through by s and taking the limit s 0 we obtain: x ( T cos θ) = ρ s t y ( T sin θ) = ρ s t (15) 1

15 x y since cosθ = and sinθ = the above equations can be reduced to the form (for s s constant T) x x T = ρ s t y y T = ρ s t (15a) Since there is no motion in the x direction x t = 0. y Note that tanθ =, therefore x sinθ tanθ y x = 1+ tan θ for small θ therefore in (15a): y y y y T = T = T sinθ T = T s x x s x x x x and the system of equations reduces to: x = 0 t y T y = t ρ x (16) The first equation has a trivial solution. The second equation can be solved by the method of separation of variables by assuming that yxt (, ) = X( xft ) ( ) which leads to: X d F T d X dt = F and dividing both sides by T XF ρ dx ρ we obtain: ρ 1 d F 1 d X = = κ TF dt X dx according to previous reasoning indicating that an equality between functions of different variables implies that the functions are equal to a constant. Therefore we can write:

16 d F T d X = κ F and = κ X dt ρ dt as previously, the solutions of these equations correspond to the case where the constant is positive and therefore the characteristic equation of the second order differential equation has imaginary coefficients, leading to the following solutions in terms of trigonometric functions: T T F = A1cosκ t+ B1sin κ t ρ ρ X = A cosκx+ B sinκx Therefore the system of equations given in (16) has product solutions of the form: T T ytx (, ) = A1cosκ t+ B1sinκ t Acosκx+ Bsinκx ρ ρ When κ = 0 then: F0 = A0 + B0t X0 = C0 + D0x y = A + B t C + D x ( )( ) ( ) Boundary conditions. Vibrating string clamped at both ends If we impose the B.C.s that the string is clamped at the ends, namely y = 0 at x = 0, L then there is no motion in the y direction and: T T ytx (, ) = A1cosκ t+ B1sinκ t A 0 ρ ρ = y0 = ( A0 + B0t) C0 = 0 T T ytx (, ) = A1cosκ t+ B1sinκ t ( AcosκL BsinκL) 0 ρ ρ + = y = A + B t C + D L = 0 ( )( ) These B.C.s are satisfied by A = C0 = D0 = 0 and the eigenvalues κl= nπ n= 1,,3,... leading to eigenfunctions: 3

17 nπx nπ T nπ T yn = sin Ancos t Bnsin t n= 1 L + L ρ L ρ (17) where the constant A 1 is now include in has a configuration that depend son on n: nπ x yn( x, t0) = Cn( t0)sin L A n and B n. Note that for any time t 0 the string which describes a family of modes of the string for the specific B.C.s of clamped ends. These are called the normal modes of vibration. The intensity of sound depends on the amplitude C = A + B which is derived from: n n n Acosθ + Bsinθ = A + B sin( θ + λ) λ = tan 1 The number of oscillations per unit time or frequency in cycles per second is: 1 nπ T n T ω = = π L ρ L ρ Sound is produced by the superposition of natural frequencies n = 1,,3,. The normal mode is the first harmonic or fundamental n = 1. The larger the natural frequency, the higher the pitch. Tuning is accomplished by varying either L, ρ or T. For vibrating strings the frequencies of the higher harmonics are all integral multiples of the fundamental. Note that sound is produced by strings vibrating in a lateral direction, however it is transmitted by waves of compression and rarefaction in the longitudinal direction (the direction of propagation). Standing waves and summation of traveling waves Each standing wave is composed by the summation of two waves traveling in opposite directions. A B Recall that: 1 1 sin xsin y = cos( x y) cos( x+ y) cos( x + y) = cos xcos y sin xsin y cos( x y) = cos xcos y+ sin xsin y Subtracting we obtain: 4

18 1 1 sin xsin y = cos( x y) cos( x+ y) Therefore any single component yj ( x, t ) of the function yn( x, t ) jπx jπ T 1 jπ T 1 jπ T yj ( x, t) = sin sin t = cos x t cos x t L L ρ L + ρ L ρ Wave traveling to Wave traveling to the left. the right. Where the wave velocity V is V T =. ρ Figure Figure shows how the equation in time is set up for advancing pulse. The upper diagram shows the pulse at t = 0 given by the equation y = f(x). The lower diagram shows the same pulse at time t = t having advanced a distance Vt without changing shape. A new axis Y is constructed, displaced a distance x = Vt to the right, x being the new coordinate of any point referred to the new origin. The equation f(x) at time t in terms of x is the same as the equation at t = 0 in terms of x, or y = f(x ) for t = t. However: x ' = x Vt therefore y = f ( x Vt) Initial conditions Suppose the I.C.s are given by a function y(x) such that for t = 0: 5

19 xd L yx (,0) = 0 x L d L yx (,0) = ( L x) x L L dy( x,0) = 0 dt Applying the velocity boundary conditions for t = 0 to (17) we obtain: dy nπ T nπx = Bn cos = 0 dt L ρ L n= 1 Which satisfies the I.C.s by setting B n = 0. Solution of the problem requires determining the constants A so that that the initial conditions are satisfied: n n= 1 nπ x d L An sin = x 0 x L L d L = ( L x) x L L multiplying both sides of (18) bysin m π x L and integrating between 0 and L we obtain: (18) L L / L nπx mπx d mπx d mπx A sin sin dx sin dx ( L x) sin dx L = + L L L L L (19) n 0 n= 1 0 L/ Integrating (19) term by term: L/ L L d mπx mπx Am = xsin dx + ( L x)sin dx L L L 0 L/ (0) These integrals can be evaluated by considering that: 1 1 a a xsin xdx = sin x x cos x and xsin axdx = axsin axdax = θ sinθdθ (1) where ax = θ. In (0), upon integration and evaluation at the limits the only non-zero terms exist at L/, of which there are two, with same sign. Therefore, in (1) we can set a = mπ / L, then 6

20 A m 8d mπ x = sin m π L which vanishes when m is even and when it is odd the sine term oscillates between the values 1 ±. The product solution therefore is: 8d mπx mπ T yxt (, ) = sin cos t m= 1,3,5,... () ρ m= 1 m π L L 7

21 BENG 1 Lecture 18 M. Intaglietta Transmission of waves in gases. Sound. Strings present the transmission of transverse waves. In gases waves are transmitted longitudinally. We will analyze the transmission of waves in tube where gas displacements are made by a piston. Moving the piston creates a compression that travels forward. If the piston is quickly retracted then there is a wave of rarefaction that also travels along the tube. Consider an element of gas in the tube located between x and x + x where the gas has an equilibrium pressure p 0. As the wave advances the element of gas oscillates about its equilibrium position. The coordinate y is used to describe displacements of gas from its equilibrium position. The displacement of the left side of the element of gas has coordinate y and that on the right side y+ y. Pressure on the left side is p and on the right side is p + p. For a very thin slice pressure in the displaced gas is p+ p 0, which is also the pressure on the left side face, and the pressure on the right side face is p+ p + p. The forces acting on the element of gas are obtained by multiplying by the 0 pa. If ρ 0 is then the mass of element is ρ A x 0 area of the tube A. The net restoring force acting on the displaced gas is the density of the gas at the equilibrium pressure leading to the equation of motion: p0 Note that x gives the position of the gas molecules at rest (therefore while p 0 is uniform in x ) while y gives the position of displaced molecules and p is not uniform in y. In the case illustrated since y > x we are dealing with a rarefaction wave. (( ) ( )) d y p+ p p+ p + p A= pa= ρ A x dt

22 d y dt 1 p = ρ x 0 and at the limit for very small x : y t 1 p = ρ x 0 (3) The volume in its equilibrium position is A x. In the displaced position the coordinate of the right face is x + x+ y+ y while the coordinate of the left face is x + y. Therefore the length of the displaced element is given by the difference of these two coordinates or x + y and the change in length, and therefore the change in volume is A y. Consider the general definition of compressibility k: 1 change in volume k = original volume change in pressure Note that the compressibility of a gas can be derived from the perfect gas equation pv = RT where dv RT 1 dv prt 1 = and k = = = dp p V dp RTp p Referring this definition to our development: 1 A y y k = = A x + p x (( p0 p) p0) therefore: 1 y p = k x and in the limit: 1 y p = k x (4) and in view of (3)

23 p 1 y = x k x Therefore substituting in (3) we are led to the one dimensional wave equation for the transmission of longitudinal perturbations: y 1 d y = t kρ dx 0 The velocity of propagation, by analogy to the wave equation for strings (lateral displacements) is given by: v = 1 kρ 0 The bulk modulus B is the reciprocal of the compressibility, in other words, the pressure required to induce a volume change relative to the total volume. This quantity is the equivalent to the Young s modulus Y for linear changes (stress required o induce a change in strain). Therefore a general expression for the velocity at which waves travel in a materials is: v = B ρ 0 Pressure variation in a sound wave From the development of the propagation velocity of lateral displacement (waves) in a string we found that a disturbance is propagated with a velocity ν, where in this case L = wave length, and A = displacement amplitude π n y = Acos ( x νt) L If we know the displacement as a function of time pressure by differentiating with respect to x since: yxt (, ) we can compute the 1 y p = k x in view of (4) which leads to: 3

24 dy π A π = sin ( x νt) dx L L and therefore: π A π p = sin ( x νt) kl L Since ν = 1 kρ 0 πρ0ν A π p = sin ( x νt) (5) L L The term within brackets represents the maximal pressure amplitude P while A is the maximal displacement. Wave velocity & thermodynamics of perfect gases The previous equation (5) shows that: p π A π = ν sin ρ0 L L (6) ( x ν t) therefore using the thermodynamic relation: M where nm ρ pv = nrt = n RT = ρ then p = RT M V M therefore in terms of maximum values (6) is also equal to: p RT = = ρ M ν a result derived by Newton, which underestimates the actual speed by about 15%. The more correct formulation was give by Laplace, who realized that the compression and relaxation in the sound wave is too rapid for allowing constant temperature (isothermal conditions), and that the actual conditions were adiabatic, i.e., no heat transfer due to the high speed at which compression and rarefaction occur, leading to the expression: ν = RT γ M 4

25 C where γ = C p v which is the ratio of specific heat at constant pressure vs. constant volume, usually about 1.4. Pressure dispersion The analytical derivation does not include a mechanism for the attenuation of the pressure amplitude, which occurs due to refraction and absorption of the pressure wave. This can be accounted for the expression: x A = Ae α o where α is a parameter that characterizes the viscous effects in the medium, or the conversion of mechanical energy in the wave into thermal energy. Pressure waves that originate from a point source decay naturally at the rate of -60 log R db, where R is the ratio of radial distances, and for cylindrical sources at the rate of -40 log R. p = K p t The dispersion of pressure can also be described by the diffusion equation: Intensity of sound waves Waves propagate energy. The intensity I of a traveling wave is defined as the average rate energy is transported by the wave per unit area across a surface perpendicular to the direction of propagation. Also intensity is the average power transported per unit area. The energy associated with a travelling wave is in part potential, associated with the compression of the medium, and kinetic relate to particle velocity. By analogy to the dynamics and energy distribution of the spring mass system, the total energy is constant in time (no dissipation) ad we can calculate intensity by considering only pressure effects. W Work done in the compression process is: = p dv and introducing the definition for compressibility: 5

26 dv kv dp W kv pdp = 0 therefore = 0 Integrating between 0 and the maximum pressure change P defines the pressure energy per unit volume, which is the same as the total energy per unit volume: W v = 0 1 kp The sound energy in the volume traveling with a wave, or energy crossing per unit area per unity time equals the energy in the volume element Aν t divided by A: I 1 P ρν = kpν = 6

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